The field of the invention relates to magnetic resonance imaging methods and systems. More particularly, the invention relates to systems and methods for performing magnetic resonance elastography (MRE) with improved efficiency by utilizing pulse sequences that are often avoided when performing MRE because the pulse sequences, though relatively expedient by nature, can introduce substantial artifacts in elastogram images. The present invention provides systems and methods to control and overcome these drawbacks to produce clinically-useful elastogram images when using such pulse sequences.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the nuclear spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. Usually the nuclear spins are comprised of hydrogen atoms, but other NMR active nuclei are occasionally used. A net magnetic moment Mz is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B1; also referred to as the radiofrequency (RF) field) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped” into the x-y plane to produce a net transverse magnetic moment Mt, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation field B1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance (“NMR”) phenomenon is exploited.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged experiences a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The emitted MR signals are detected using a receiver coil. The MRI signals are then digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
It has been found that MR imaging can be enhanced when an oscillating stress is applied to the object being imaged in a method called MR elastography (MRE). MRE is gaining wider clinical applicability due to its ability to noninvasively and quantitatively measure tissue stiffness. MRE is a multi-step process beginning with the induction of shear waves in the tissue to be examined via an external source of vibration. The shear waves are then imaged with a phase-contrast MRI pulse sequence with motion-encoding gradients synchronized with the applied vibration. The resulting wave images of the wave motion are inverted to calculate the tissue stiffness and produce an elastogram image.
MRE is analogous to manual palpation, which has a long history in the practice of medicine as a clinical diagnostic tool for examining tissues such as the breast and thyroid for focal and diffuse diseases. In fact, MRE of the liver has already matured to a point where it is replacing needle biopsies for the diagnosis of fibrosis and cirrhosis in a growing number of clinical practices.
As generally described above, MRE utilizes the oscillating stress produced by the shear waves that propagate through the organ, or tissues to be imaged, to elicit information about tissue stiffness. Specifically, these shear waves alter the phase of the MR signals and, from this, the mechanical properties of the tissue can be determined. However, to do so, the MRI pulse sequence must be carefully timed to the oscillations generating the shear waves. That is, the wave images are calculated as the phase difference between two images that are acquired using an MRI pulse sequence. Specifically, one image is acquired during a positive motion encoding gradients and the second image is acquired during a negative motion encoding. The two motion encoding gradients are synchronized to the oscillations used to generate the shear waves.
Unfortunately, hardware or software errors in the acquisition process can result in a constant or slowly varying phase ramp that may remain in the wave image after performing the phase difference calculation. For example, some very popular and advantageous (for example, fast) pulse sequences, such as the echo-planar imaging (EPI) pulse sequence, inherently present such errors when used as part of an MRE pulse sequence. To make matters worse, in some instances, these errors can vary over time. Further still, when performing three-dimensional (3D) image processing, these errors or discontinuities can produce a high frequency artifact in the slice direction that results in an inaccurate stiffness calculation. When manifested as inaccurate stiffness calculations, such errors can undermine the clinical utility of the final elastogram image. Such erroneous stiffness calculations, in some cases, can not be readily discerned by the clinician, even when highly experienced in reading stiffness calculations or elastogram images. As such, though there are a variety of pulse sequences that are advantageously and regularly used in other MRI applications, such as the EPI pulse sequence, such pulse sequences are often foregone when performing MRE.
Therefore, it would be desirable to have a system and method for expanding the variety of pulse sequences available when performing MRE, particularly, to include pulse sequences such as EPI that are generally regarded as highly efficient and versatile.